This invention relates to backseats for stem valve assembles, and more particularly to such backseats that provide a metal-to-metal seal between the valve stem and bonnet.
Backseats are added to valve stems as a safety feature to provide a backup stem sealing mechanism for the packing that functions as the primary or main seal between the stem and the valve bonnet. If the packing fails, such as by developing a leak from wear, exposure to hostile fluids in the valve, exposure to extreme hot or cold temperatures, or any combination of these, the metal-to-metal seal created by the backseat is the only barrier left to prevent escape of fluid from the valve. Packings are especially vulnerable to destruction by fire, and in the absence of a backseat the escaping fluid could ignite and thus contribute to the fire's magnitude.
Conventional valve backseats employ either matched or non-matched angles of contact between the backseat and the bonnet. The conventional backseat is located on a relatively thick and non-flexible enlarged diameter portion of the valve stem, and therefore will not compensate for any mis-alignment between the stem and bonnet. The sealing ability of the conventional backseat design is very good when the high pressure inside the valve body acts on the end of the valve stem, resulting in sufficient axial load on the stem to prevent leakage. This design, however, will eventually begin to leak as the pressure in the valve body decreases and, therefore, the load on the valve stem also decreases. To overcome this drawback the stem is mechanically loaded to produce a seal at low pressures. Actuated valves, incorporating a spring return device, often will have a larger than standard spring to achieve this necessary force. The disadvantage of this approach is that the force required to create a seal with a conventional backseat design is quite high, often in excess of 2000 pounds. The spring required to produce such a force, and the larger sized actuator piston needed to overcome that spring force, causes the actuator to be significantly large and much more expensive.